How can I find out if I'll receive a 60-day or 30-day visa exemption for my 5-month stay in Thailand?

There is no way of knowing what and when it will change. If you need more sevurity then apply for a tourist visa.

Coming to Thailand and expecting anything to be certain in advance is a forlorn hope. Try Singapore !

As things stand you will get 60 days, there have been rumours of it changing to 30 but nothing official. But if you're planning 5 months you may want to apply for a multiple entry tourist visa before you travel. That's good for 6 months with one border run and one extension you get 5 months

You won't know about visa exemption but you could just apply for a tourist visa in your own country before you arrive that was you are guaranteed to get your 60 days then extend in country for another 30 days, do a visa run and if it's only 30 days visa exempt then you can extend that again for 30 days meaning you'll be ok.

No way of knowing.